In Days Gone, you can use oil filters to suppress your firearms. Mounting issue aside, is this something that would actually work to any practical degree?

__________________"The gun has played a critical role in history. An invention which has been praised and denounced... served hero and villain alike... and carries with it moral responsibility. To understand the gun is to better understand history."

“What is morality in any given time or place? It is what the majority then and there happen to like, and immorality is what they dislike.” - Alfred North Whitehead

They are a real thing. A few years ago there was a fad of companies selling thread adapters that allowed you to thread an oil filter onto a muzzle. The ATF obviously had a problem with this as they were not selling them as "suppressors" but were instead trying selling them as "solvent traps" for collecting excess oil after cleaning your gun. The absurdity of this reason didn't hold water for long and the ATF deemed the adapter to be a "suppressor" so most of them went away. I think that a few people do still make them as registered suppressors with the idea being that they are cheaper and the only registered part will last forever and the unregistered oil filters can be infinitely replaced. As for effectiveness, they will obviously not be as good as a real one but from what I have heard they do work fairly well.

About fifteen years ago I was involved in a murder investigation. The killer used a plastic bottle to suppress the sound of the gunshots. He shot the victim three times in the back as he was working on his computer. However the victim didn't die immediately. He called 9-11 and gave dispatch the name of his killer. He also told the responding officers, the paramedics. the doctors and nurses in the emergency room. Deathbed statements are admissible in court. The shooter owed him money for drugs. He tried to deny it, but we had his prints on the bottle and empty casings that we matched to his pistol. He later plead guilty and went to prison for 1st degree murder. However the improvised suppresser really did work. Probably helped that it was a 22 LR pistol. He learned about improvised suppressors from the Internet. I'm not kidding.